Binary Encodings for JavaScript Object Notation: JSON-B, JSON-C, JSON-D
draft-hallambaker-jsonbcd-03

Abstract

Three binary encodings for JavaScript Object Notation (JSON) are presented. JSON-B (Binary) is a strict superset of the JSON encoding that permits efficient binary encoding of intrinsic JavaScript data types. JSON-C (Compact) is a strict superset of JSON-B that supports compact representation of repeated data strings with short numeric codes. JSON-D (Data) supports additional binary data types for integer and floating point representations for use in scientific applications where conversion between binary and decimal representations would cause a loss of precision.

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1. Definitions

1.1. Requirements Language"

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. Introduction

JavaScript Object Notation (JSON) is a simple text encoding for the JavaScript Data model that has found wide application beyond its original field of use. In particular JSON has rapidly become a preferred encoding for Web Services.

JSON encoding supports just four fundamental data types (integer, floating point, string and boolean), arrays and objects which consist of a list of tag-value pairs.

Although the JSON encoding is sufficient for many purposes it is not always efficient. In particular there is no efficient representation for blocks of binary data. Use of base64 encoding increases data volume by 33%. This overhead increases exponentially in applications where nested binary encodings are required making use of JSON encoding unsatisfactory in cryptographic applications where nested binary structures are frequently required.

Another source of inefficiency in JSON encoding is the repeated occurrence of object tags. A JSON encoding containing an array of a hundred objects such as {"first":1,"second":2} will contain a hundred occurrences of the string "first" (seven bytes) and a hundred occurrences of the string "second" (eight bytes). Using two byte code sequences in place of strings allows a saving of 11 bytes per object without loss of information, a saving of 50%.

A third objection to the use of JSON encoding is that floating point numbers can only be represented in decimal form and this necessarily involves a loss of precision when converting between binary and decimal representations. While such issues are rarely important in network applications they can be critical in scientific applications. It is not acceptable for saving and restoring a data set to change the result of a calculation.

2.1. Objectives

The following were identified as core objectives for a binary JSON encoding:

Low overhead encoding and decoding
Easy to convert existing encoders and decoders to add binary support
Efficient encoding of binary data
Ability to convert from JSON to binary encoding in a streaming mode (i.e. without reading the entire binary data block before beginning encoding.
Lossless encoding of JavaScript data types
The ability to support JSON tag compression and extended data types are considered desirable but not essential for typical network applications.

Three binary encodings are defined:

JSON-B (Binary): Simply encodes JSON data in binary. Only the JavaScript data model is supported (i.e. atomic types are integers, double or string). Integers may be 8, 16, 32 or 64 bits either signed or unsigned. Floating points are IEEE 754 binary64 format [!IEEE-754]. Supports chunked encoding for binary and UTF-8 string types.
JSON-C (Compact): As JSON-B but with support for representing JSON tags in numeric code form (16 bit code space). This is done for both compact encoding and to allow simplification of encoders/decoders in constrained environments. Codes may be defined inline or by reference to a known dictionary of codes referenced via a digest value.
JSON-D (Data): As JSON-C but with support for representing additional data types without loss of precision. In particular other IEEE 754 floating point formats, both binary and decimal and Intel's 80 bit floating point, plus 128 bit integers and bignum integers.

3. title="Extended JSON Grammar">

The JSON-B, JSON-C and JSON-D encodings are all based on the JSON grammar [RFC4627] /> using the same syntactic structure but different lexical encodings.

JSON-B0 and JSON-C0 replace the JSON lexical encodings for strings and numbers with binary encodings. JSON-B1 and JSON-C1 allow either lexical encoding to be used. Thus any valid JSON encoding is a valid JSON-B1 or JSON-C1 encoding.

The grammar of JSON-B, JSON-C and JSON-D is a superset of the JSON grammar. The following productions are added to the grammar:

x-value">: Binary encodings for data values. As the binary value encodings are all self delimiting
x-member: An object member where the value is specified as an X-value and thus does not require a value-separator.
b-value: Binary data encodings defined in JSON-B.
b-string: Defined length string encoding defined in JSON-B.
c-def: Tag code definition defined in JSON-C. These may only appear before the beginning of an Object or Array and before any preceeding white space.
c-tag: Tag code value defined in JSON-C.
d-value: Additional binary data encodings defined in JSON-D for use in scientific data applications.

The JSON grammar is modified to permit the use of x-value productions in place of ( value value-separator ) :

JSON-text = (object / array)

object = *cdef begin-object [
*( member value-separator | x-member )
(member | x-member) ] end-object

member = tag value
x-member = tag x-value

tag = string name-separator | b-string | c-tag

array = *cdef begin-array [  *( value value-separator | x-value )
(value | x-value) ] end-array

x-value = b-value / d-value

value = false / null / true / object / array / number / string

name-separator  = ws %x3A ws  ; : colon
value-separator = ws %x2C ws  ; , comma

The following lexical values are unchanged:

begin-array     = ws %x5B ws  ; [ left square bracket
begin-object    = ws %x7B ws  ; { left curly bracket
end-array       = ws %x5D ws  ; ] right square bracket
end-object      = ws %x7D ws  ; } right curly bracket

ws = *( %x20 %x09 %x0A  %x0D )

false = %x66.61.6c.73.65   ; false
null  = %x6e.75.6c.6c      ; null
true  = %x74.72.75.65      ; true

The productions number and string are defined as before:

number = [ minus ] int [ frac ] [ exp ]
decimal-point = %x2E       ; .
digit1-9 = %x31-39         ; 1-9
e = %x65 / %x45            ; e E
exp = e [ minus / plus ] 1*DIGIT
frac = decimal-point 1*DIGIT
int = zero / ( digit1-9 *DIGIT )
minus = %x2D               ; -
plus = %x2B                ; +
zero = %x30                ; 0

string = quotation-mark *char quotation-mark
char = unescaped /
escape ( %x22 / %x5C / %x2F / %x62 / %x66 /
%x6E / %x72 / %x74 /  %x75 4HEXDIG )

escape = %x5C              ; \
quotation-mark = %x22      ; "
unescaped = %x20-21 / %x23-5B / %x5D-10FFFF

4. JSON-B

The JSON-B encoding defines the b-value and b-string productions:

b-value = b-atom | b-string | b-data | b-integer |
b-float

b-string = *( string-chunk ) string-term
b-data = *( data-chunk ) data-last

b-integer = p-int8 | p-int16 | p-int32 | p-int64 | p-bignum16 |
n-int8 | n-int16 | n-int32 | n-int64 | n-bignum16

b-float = binary64

The lexical encodings of the productions are defined in the following table where the column 'tag' specifies the byte code that begins the production, 'Fixed' specifies the number of data bytes that follow and 'Length' specifies the number of bytes used to define the length of a variable length field following the data bytes:

Production	Tag	Fixed	Length	Data Description
string-term	x80	-	1	Terminal String 8 bit length
string-term	x81	-	2	Terminal String 16 bit length
string-term	x82	-	4	Terminal String 32 bit length
string-term	x83	-	8	Terminal String 64 bit length
string-chunk	x84	-	1	Non-Terminal String 8 bit length
string-chunk	x85	-	2	Non-Terminal String 16 bit length
string-chunk	x86	-	4	Non-Terminal String 32 bit length
string-chunk	x87	-	8	Non-Terminal String 64 bit length
data-term	x88	-	1	Terminal Data 8 bit length
data-term	x89	-	2	Terminal Data 16 bit length
data-term	x8A	-	4	Terminal Data 32 bit length
data-term	x8B	-	8	Terminal Data 64 bit length
data-chunk	x8C	-	1	Non-Terminal Data 8 bit length
data-chunk	x8D	-	2	Non-Terminal Data 16 bit length
data-chunk	x8E	-	4	Non-Terminal Data 32 bit length
data-chunk	x8F	-	8	Non-Terminal String 64 bit length
p-int8	xA0	1	-	Positive 8 bit Integer
p-int16	xA1	2	-	Positive 16 bit Integer
p-int32	xA2	4	-	Positive 32 bit Integer
p-int64	xA3	8	-	Positive 64 bit Integer
p-bignum16	xA5	-	2	Positive Bignum 16 bit length
n-int8	xA8	1	-	Negative 8 bit Integer
n-int16	xA9	2	-	Negative 16 bit Integer
n-int32	xAA	4	-	Negative 32 bit Integer
n-int64	xAB	8	-	Negative 64 bit Integer
n-bignum16	xAD	-	2	Negative Bignum 16 bit length
binary64	x92	8	-	IEEE 754 Floating Point binary64
b-value	xB0	-	-	True
b-value	xB1	-	-	False
b-value	xB2	-	-	Null

A data type commonly used in networking that is not defined in this scheme is a datetime representation.

4.1. JSON-B Examples

The following examples show examples of using JSON-B encoding:

Binary Encoding                  JSON Equivalent

A0 2A                            42 (as 8 bit integer)
A1 00 2A                         42 (as 16 bit integer)
A2 00 00 00 2A                   42 (as 32 bit integer)
A3 00 00 00 00 00 00 00 2A       42 (as 64 bit integer)
A5 00 01 42                      42 (as Bignum)

80 05 48 65 6c 6c 6f             "Hello" (single chunk)
81 00 05 48 65 6c 6c 6f          "Hello" (single chunk)
84 05 48 65 6c 6c 6f 80 00       "Hello" (as two chunks)

92 3f f0 00 00 00 00 00 00       1.0
92 40 24 00 00 00 00 00 00       10.0
92 40 09 21 fb 54 44 2e ea       3.14159265359
92 bf f0 00 00 00 00 00 00       -1.0

B0                               true
B1                               false
B2                               null

5. JSON-C

JSON-C (Compressed) permits numeric code values to be substituted for strings and binary data. Tag codes MAY be 8, 16 or 32 bits long encoded in network byte order.

Tag codes MUST be defined before they are referenced. A Tag code MAY be defined before the corresponding data or string value is used or at the same time that it is used.

A dictionary is a list of tag code definitions. An encoding MAY incorporate definitions from a dictionary using the dict-hash production. The dict hash production specifies a (positive) offset value to be added to the entries in the dictionary and a hash code identifier consisting of the ASN.1 OID value sequence for the cryptographic digest used to compute the hash value followed by the hash value in network byte order.

Production	Tag	Fixed	Length	Data Description
c-tag	xC0	1	-	8 bit tag code
c-tag	xC1	2	-	16 bit tag code
c-tag	xC2	4	-	32 bit tag code
c-def	xC4	1	-	8 bit tag definition
c-def	xC5	2	-	16 bit tag definition
c-def	xC6	4	-	32 bit tag definition
c-tag	xC8	1	-	8 bit tag code & definition
c-tag	xC9	2	-	16 bit tag code & definition
c-tag	xCA	4	-	32 bit tag code & definition
c-def	xCC	1	-	8 bit tag dictionary definition
c-def	xCD	2	-	16 bit tag dictionary definition
c-def	xCE	4	-	32 bit tag dictionary definition
dict-hash	xD0	4	1	Hash of dictionary

All integer values are encoded in Network Byte Order (most significant byte first).

5.1. JSON-C Examples

The following examples show examples of using JSON-C encoding:

JSON-C                           Value      Define

C8 20 80 05 48 65 6c 6c 6f       "Hello"    20 = "Hello"
C4 21 80 05 48 65 6c 6c 6f                  21 = "Hello"
C0 20                            "Hello"
C1 00 20                         "Hello"

D0 00 00 01 00 1B                           277 = "Hello"
06 09 60 86 48 01 65 03
04 02 01                      OID for SHA-2-256
e3 b0 c4 42 98 fc 1c 14
9a fb f4 c8 99 6f b9 24
27 ae 41 e4 64 9b 93 4c
a4 95 99 1b 78 52 b8 55       SHA-256(C4 21 80 05 48 65 6c 6c 6f)

2.16.840.1.101.3.4.2.1

6. JSON-D (Data)

JSON-B and JSON-C only support the two numeric types defined in the JavaScript data model: Integers and 64 bit floating point values. JSON-D (Data) defines binary encodings for additional data types that are commonly used in scientific applications. These comprise positive and negative 128 bit integers, six additional floating point representations defined by IEEE 754 [RFC2119] and the Intel extended precision 80 bit floating point representation.

Should the need arise, even bigger bignums could be defined with the length specified as a 32 bit value permitting bignums of up to 2^35 bits to be represented.

d-value = d-integer | d-float

d-float = binary16 | binary32 | binary128 | binary80 |
decimal32 | decimal64 | decimal 128

Production	Tag	Fixed	Length	Data Description
p-int128	xA4	16	-	Positive 128 bit Integer
n-in7128	xAC	16	-	Negative 128 bit Integer
binary16	x90	2	-	IEEE 754 Floating Point binary16
binary32	x91	4	-	IEEE 754 Floating Point binary32
binary128	x94	16	-	IEEE 754 Floating Point binary128
intel80	x95	10	-	Intel 80 bit extended binary Floating Point
decimal32	x96	4	-	IEEE 754 Floating Point decimal32
decimal64	x97	8	-	IEEE 754 Floating Point decimal64
decimal128	x98	18	-	IEEE 754 Floating Point decimal128

7. title="Acknowledgements">

Nico Williams, etc

8. title="Security Considerations">

TBS

9. title="IANA Considerations">

[TBS list out all the code points that require an IANA registration]

10. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4627]	Crockford, D., "The application/json Media Type for JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[IEEE-754]	, , "[Reference Not Found!]"

Author's Address

Phillip Hallam-Baker Comodo Group Inc. EMail: philliph@comodo.com

1. Definitions

1.1. Requirements Language"

2. Introduction

2.1. Objectives

3. title="Extended JSON Grammar">
4. JSON-B

4.1. JSON-B Examples

5. JSON-C

5.1. JSON-C Examples

6. JSON-D (Data)
7. title="Acknowledgements">
8. title="Security Considerations">
9. title="IANA Considerations">
10. Normative References
Author's Address